class_docstr: Callback to implement iteration based model averaging strategies.
  
  Parameters
  ----------
  avg_strategy
      IterationAveragingStrategy, one of NTA or Alpha_Suffix from
      gluonts.mx.trainer.model_iteration_averaging
code: def on_validation_epoch_start(
      self, training_network: nn.HybridBlock
  ) -> None:
      # use averaged model for validation
      self.avg_strategy.load_averaged_model(training_network)
  
imports: from typing import Any, Dict, List, Optional
  import logging
  import mxnet
  import mxnet.gluon.nn
  from mxnet import gluon
  from gluonts.core.component import validated
  from .callback import Callback
label_desc: Use image node_id 2 for calling the ModelIterationAveraging obj's underlying member method code with example usage: obj.on_validation_epoch_start(training_network) without return types